Spectrometer system

ABSTRACT

A wobbling single slit is provided as the entrance slit of a spectrometer which rectilinearly reciprocates at a constant amplitude, such amplitude being selectively adjustable. A control signal is obtained from a circuit that oscillates the slit for controlling the frequency and phase of the ac signal component detected and amplified from the detector after the superposed dc detected signal component is separated from the ac signal component. The invention described herein was made in the course of work under a grant or award from the Department of Health, Education and Welfare.

United States Patent [1 1 m1 3,756,721 Williams Sept. 4, 1973SPECTROMETER SYSTEM 5 the Amplitude of a Mechanical Oscillator", Masters75 Inventor: David T. Williams, Gainesville, Fla. Thesis University 9Emma 1968- Williams et al.: Molecular Correlation Spectrometry",

[73] Asslgnee. Board of Regents, State of Florlda, Applied Optics VOL 7,No. 4 Apr 1968 pages Tallahassee, Fla.

[22] Filed: Mar. 6, 1972 [211 App] No 232 053 Primary Examiner-Ronald L.Wibert Assistant ExaminerF. L. Evans Related U.S. Application Data 7Afio rfiy Ii rfifl35lfififind Arthur G. Yeager [63] Continuation-impartof Ser. No. 59,456, July Q 1970, abandoned. v v 521 U.S. Cl 356/80,356/97, 356/98 A Wobbling Single Slit is provided as the entrance Slitof 51 int. Cl G0lj 3/04, G01 j 3/42 a Spectrometer whichrectilinearlyreciprocates at a 58 Field of Search ..356/74-84, 88, 89,93-101 constant amplitude, such amplitude being selectively adjustable.A control signal is obtained from a circuit 7 that oscillates the slitfor controlling the frequency and 5 R f n Cited phase of the ac signalcomponent detected and ampli- UNITED STATES PATENTS f ed from thedetector after the superposed dc detected signal component ls separatedfrom the ac slgnal com- 2,823,577 2/1958 Machler 356/82 3,520,614 7/1970Goldstein.... '356/97 f I 3,482,105 12/1969 Hutzler 356/97 X Thelnventlon described herein was made in the course 3,518,002 6/1970Barringer et al. 356/97 of work under a grant or award from theDepartment Of Health, Education and Welfare.

Harris: Design of a Feedback Circuit for Controlling v i 12 Claims, 3Drawing Figures chopper 27 f'z'lzer 30 77 4! 45, 55,,

\( D Spectrometer-14 p l 21);,

l Tuned 12 f g Y Zfiflmp/ifier 13 l j 1 050mm 691/16 17 33/ i I 1!,Syuarprg 1 Wit/er L- I Driller ifl I Phase S/zrffzr 28 freymwcy v fl az/fi/er SPECTROMETER SYSTEM I This application is a continuation-in-partof application Ser. No. 59,456, filed July 30, 1970, now abandoned.

1. Field of the Invention This invention relates to spectroscopy andmore parof the various absorption coefficients of the gas at thepositions of the slits in the mask. If the frequency of the motion ofthe spectrum relative to the mask is n, then the signal will beincreased at all points out of register, and an ac signal will begenerated of a frequency pn where p is some whole number less than 10.This ac signal can be amplified by use of a tuned amplifier; and

Various spectrometer systems have been proposed by many others amongwhich are the following: A. Barringer, Paper 67/196, 60th Annual AirPollution Control Association meeting; B. Williams, et al., MolecularCorrelation Spectrometry. Appl. Opt. 7 (4) 607-16 (1968); C. Design of aFeedback circuit for controlling the amplitude of a mechanicaloscillator", l-Iarris, Masters thesis, University of Florida, (1968); D.Giese et al, Appl. Spectroscopy 9, 7896 (1955); E. Collier et al, J.Appl. Chem. 6 495-510 (1956); F. Pemsler, RSI 28 (4) 274 and 5 (1957);G. Olson et al, Analyt. Chem 32 (3) 370-373 (1960); H. Balslev, Phys.Rev. 143 (2) 636-647 (1966); I. Arumu et al, RSI 37 (12) 1696-1698(1966); J. Bonfiglioli et al, Appl. Opt. 3 (12) 1417-1424 (1965); K.Bonfiglio et al, Appl. Opt. 6 (3) 447-455 (1967); L. Gilgore et al, RSI38, 1535 and 6 (1967); M. Overend et al, Appl. Opt. 6 (3) 457-466(1967); N. Perregaux et al, Appl. Opt. 7 (10) 2031-2035 (1968); and O.Stauffer et al, Appl. Opt. 7 (1) 61-65 (1968).

References A, C, L. N. and 0. above describe different techniques andarrangements of moving the radiant energy beam in the spectrometersystem and are in general more directly related to the present inventionthan the other references cited hereinabove.

In principle the present invention relates essentially to an improvedspectrometer system for comparing the intensity of light of a particularwavelength with the intensity of light of wavelengths immediatelyadjacent to that wavelength in the same spectrum. In that sense there isa resemblance of this system to the general art of light choppers" inwhich two different sources of light either of the same or of differentwavelengths may be compared many times a second as well known in thefield of infrared spectroscopy. Another arrangement .is disclosed byBarringer et al in Reference A above. In their system light from acontinuous source is introduced through some known or assumed length Lof a path containing a concentration c of a gas which is to be measured.The light is dispersed in the conventional manner in the spectrometer soas to produce an array of more or less overlapping images of theentrance slit in the colors characteristic of the light source asmodified by absorption due to thegas in the amount cL. Their system hasa correlation mask set at the focal plane of the spectrometer, whichmask is an array of slits of spacing corresponding tothe absorption.spectrum of the absorbing gas to be investigated. The spectrum isoscillated torsionally to displace the entrance slit image from side toside with respect to the mask, across the point of register. At thepoint of register, the light transmitted through the mask, and thence tothe light detector, has a minimum value of a magnitude that depends onthe product cLp. where p. is a function the amplified ac signal, whenthemean output is maintained constant by use of AGC, will result in ameasure of the quantity cL of the gas. Barringer et al assert that theirarrengement has some degree of specificity in the detection of the gascorresponding to the mask, al though the arrangement is designed todetect only one gas at a time, other gases requiring the use of otherdifferently slitted masks, which normally can only be installed at thefactory.

SUMMARY OF THE INVENTION 7 In accord with various aspects of theinvention a wobbling slit means in the form of a single slit in amovable carrier is provided in the radiant energy path between theradiant energy source and the spectrometer. A photomultiplier detectorand an absorption cell containing a medium to be investigatedcommunicate with the radiant energy path between the spectrometer anddetector, the detector including a photomultiplier tube having gridwires. The wobbling slit is disposed innoninterferring relation to thegrid wires of the tube. Means are provided for rectilinearlyreciprocating the carrier which includes circuit means for maintainingthe amplitude of the reciprocating motion of frequency constant andselectively adjustable means for altering the amplitude. The detectorreceives the radiant energy after passage through the absorbing mediumto be investigated and converts the received radiant energy into asignal. A tune phased lock-in amplifier means receives the signal andconverts same into a first ac signal of selected frequency and a dcsignal, the first and dc signals being coupled to a potentiometricrecorder to record the ratio of the first signal to the dc signal. Thecircuit means also provides a control signal representative of thefrequency and phase of the oscillating slit motion to the amplifiermeans for use in rectifying the first signal.

The present invention provides an improved spectrometer system in whichthe motion of the spectrum is provided by a single slit wobbling maskset in front of and functioning as the entrance slit of thespectrometer, and the received ac signal with superposed dc signalisseparated and the ac component thereafter amplified in a tuned amplifierand divided by the dc component, thereby rendering the ratio output ofthe system to be independent of the fluctuations in the intensity of theradiant energy source.

The primary advantages of the present invention are that a single slitis used, rather than an array of slits, as

the mask; a wobbling system is employed to move the slit; the amplitudeof oscillation of the wobbling system.

is adjusted. at will, and provided to be constant, once set; the acsignal from the photomultiplier detector is separated from the dcsignal, and one particular frequency is amplified by use of a tunedamplifier, the amplifier output being rectified by use of a synchronousswitching ciruit, and then divided by the dc component of the originalphotomultiplier detector signal, by use of a potentiometric recorderwhereby the ratio of the recorded output is independent of the intensityand drift of the radiant energy source; the wobbling slit is mounted atthe position of the entrance slit of the spectrometer in a manner as torender unaltered the operation of the spectrometer, and to permit use ofthe spectrometer, after suitable switching adjustments, either as aconventional transmission spectrometer or a derivative spectrometer; andthe location of the wobbling slit at the position of the entrance slitmaintains the light pattern due to the exit slit stationary withrelation to the grid wires in the photomultiplier tube of the detector,which eliminates a large zero signal and instrument adjustments thatotherwise would be produced if the wobbling slit were positioned as theexit slit of the spec-. trometer, for example.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are believedto be characteristic of this invention are set forth with particularityin the appended claims. The invention itself, however, both as to itsorganization and method of operation, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawing in which:

FIG. 1 is a diagrammatic view of the spectrometer system in accord withthe present invention;

FIG. 2 is a pictorial view, partly broken away, of the wobbling entranceslit for the spectrometer of FIG. 1; and

FIG. 3 is a circuit diagram of the wobbler driver for driving theentrance slit of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularlyto the drawing of FIG. 1, the spectrometer system is generally indicatedby reference numeral and includes a radiant energy source 1 1 and amirror 12 reflecting the radiant energy through a wobbling slit means 13and into spectrometer 14 which may be of the type of a small Jarrell-Ashquarter-meter grating insturument, model 82-405, marketed under the nameMinichromator, manufactured by Jarrell-Ash Co. of Waltham, Mass. Theradiant energy passes through the slit, hereinafter more fully describedin conneith FIG. 2, which serves as or functions as the entrance slit ofthe spectrometer 14. The radiant energy is dispersed by the spectrometer14 prior to passage thereof out the exit slit 15 thereof into theabsorption cell 16 containing a medium to be investigated, such mediumnormally being gaseous with pollutants therein.

The radiant energy not absorbed, exists from cell 16, is detected by adetector 17, and is thereafter analyzed. The detector 17 includes aphotomultiplier tube 18, or the like, which normally includes aplurality of grid wires 19 therein. Since the exit slit 15 of thespectrometer 14 is stationary, the radiant energy pattern isstaterrelation of such wobbling exit slit with respect to thephotomultipler grid wires.

The signal from the detector 17, consisting of a dc signal with the acsignal superposed, is supplied to a phase lock-in amplifier meansindicated at 20 with a coupled potentiometric recorder 21 to make arecord proportional to the ratio of the ac signal to the dc signal. Byappropriate studies of the particular record various characteristics ofthe medium under investigation in the cell 16 may be determined bymethods and means well known to those skilled in the art.

The signal from detector 17 is amplified by ac-dc amplifier 22 with thedc signal component being ap-.

plied through line 23 to the potentiometric recorder 21 referencevoltage input 24. The ac signal component is passed through a couplingcapacitor 25 to a tuned amplifier 26 which may be tuned to the samefrequency as that of the wobbling slit means, hereinafter more particularly described, or to a harmonic thereof. If the frequency of thetuned amplifier 26 is the fundamental of the frequency of the wobblingslit means, the system 10 functions as a first-derivative spectrometersystem. However, when the tuned frequency is twice the frequency of suchwobbling slit means, the system 10 functions as a second-derivativespectromecter having sensitivities and capabilities of more accuratelyidentifying, for example, various pullutants in a sample to beinvestigated, of from two to four or more orders of magnitude moresensitivity that that normally achievable.

The tuned ac signal component is fed into a chopper 27, also tuned tothe second harmonic, which rectifies the ac signal component suppliedthereto, chopper 27 being controlled by a control signal from wobblingslit means 13 through a phase shifter 28 to adjust the phase of thecontrol signal and to a frequency doubler 29 and squarer 32 to developthe proper control signal or pulses which activate chopper 27. The.output signal from chopper 27 is filtered by filter 30 before suchsignal is coupled to the input 31 of potentiometric recorder 21.

The wobbling slit means is seen to include a carrier 35 carrying asingle slit 36 which functions as the entrance slit of the spectrometer14. The carrier 35 is attached to a support 37, support 37 being rigidlyconnected to a movable member 38. A pair of flat leaf springs 39 areclamped to the spring base 40 as illustrated and are also clamped tomovable member 38 spaced above base 40. Base 40 is rigidly affixed to astationary plate 41 which has an opening (not shown) therethrough forthe passage of the radiant energy from source 11 and mirror 12therethrough and through entrance slit 36 of the spectrometer 14. A rod42 is connected between member 38 and an elongated permanent magnet 43with a pair of solenoids 44 and 45 encircling respective-ends 46-and 47of magnet 43.

It is thus seen that wobbling: slit means 13 includes a carrier 35having a single slit 36 therethrough and means for oscillating suchcarrier for reciprocating motion, such latter means hereinshown asincluding the pair of leaf springs 39, as well as the other structuralcomponents, as described in connection with FIG. 1 together with awobbler driver circuit 50 as shown specifically in FIG. 3.

When the wobbler driver circuit 50 is inoperative, the pair of leafpsrings 39 maintain the slit 36 in a null position with the respectivepoles 46 and 47 of the bar magnet 43 being in the center of the solenoidcoils 44 and 45. Upon displacement of the magnet 43 further into eithercoils 44 and 45, the leaf springs 39 tend to retard such displacementand exert restoring forces substantially proportional to thedisplacement.

Assume that solenoid coil 44 is the driving coil and that this coilcauses magnet 43 to be driven further into solenoid pick-up coil 45, themovement of magnet 43 also moving the coupled slit carrier 35. A currentis induced into coil 45 by the movement of the magnet 43 axiallythereinto which essentially is in the form of a sine wave that iscoupled throuch capacitor 52 to the base 53 of tranistor 54 with properbias voltage being supplied from source 55 through resistors 68 and 69.The transistor 54 is connected essentially as a commonemitter circuit,generally analogous to a groundedcathode amplifier, which amplifies thesignal applied to base 53 without appreciable distortion. The amplifiedsignal is coupled from collector 57 through capacitor 58 to the base 59of another transistor 60 for power amplification of the signal which isin turn coupled from collector 61 to driving coil 44.

However, the springs 39 haveheretofore restored the magnet to its nullposition or slightly beyond with the magnet 43 being within the coil 44to a greater extent than at the null position. The current in the coil44 from collector 61 of transistor 60 again drives the magnet 43 backinto coil 45 and the oscillation and circuit operation continues aspreviously described.

The bias at the base59 of transistor 60 is variable by employing apotentiometer 65 which is adjusted to chop the signal supplied to thebase 59 either at saturation or cut-off of transistor 60. The currenttransmitted through the collector 61 of transistor 60 into the solenoidcoil 44 is a strong function of the amplitude of the wobbler slit peakto peak displacement, so that when potentiometer 65 is so adjusted andset, the amplitude of the wobbler slit is substantially constant-and isunchanging with respect to time. The circuit details of FIG. 3 may bemodified without departing from the spirit or scope of this invention bypersons skilled in the art, it being important to supply a non-linearcircuit element in such wobbler driver 50 to control the amplitude ofoscillation of the wobbler slit.

The spectrometer system as described above does resemble the systems ofBarringer and Harris set forth hereinabove, but the similarities arequite superficial, and the prior art systems do not achieve the resultsobtained by the instant invention nor the many advantages affordedthereby. The most important distinction is the use of a single wobblerentrance slit 13 in the instant system without the use of any type ofcorrelation mask as shown and described in Barringer and Harris.

The single wobbler entrance slit l3 in accord with this inventionrectilinearly reciprocates and causes a sinusoidal variation of thewavelerigthof the light detected by the photomultiplier tube 18. Lightchoppers used in conventional infrared technology usually compare a pairof independent light sources, which are very much more independent thanthe light of wavelengths immediately proximate in the spectrum, asemployed in the instant system. When two independent sources arecompared by use of the conventional light chopper systems, the observeddifferences in such sources have magnitudes that show a randomdistribution about a mean value, the observed differences being somewhatmore random than the largest fluctuation or variant of either of the twosources. However in the spectrometer system in accord with thisinvention in using a single wobbler entrance slit 13, the differencesbeing measured are the differences in the transmission of closelyneighboring wavelengths in a given spectrum of the light after passagethrough the absorption cell 16, and the differences are substantiallyindependent of fluctuations or variations in the light source 11, or ofthe transmission characteristics of the absorbing medium through whichthe beam passes. Any fluctuation or variation in the intensity of thelight source 11, which may be caused by voltage fluctuations, sourcedepletion, etc., or of the transmission characteristics will effectallof the entire wavelengths of light being observed at the same time.Thus, the measured difference is free of such variations or fluctuationsas would be observed it the spectral regions compared werewhollyindependent of each other.

The use of a single wobbler entrance slit 13 in accord with theinvention provides at least two important advantages and results notpreviously achievable in either the systems of Barringer or Harris, orin any other prior art known to applicant. First the concentration ofany and all gases having line absorption spectra in the range ofsensitivity of the instant system can be observed and measured withoutthe necessity of changing correlation masks or the like. The observationof any and all spectra is. performed by merely slowly rotating thediffraction grating as in the wavelength scan of theconventionalJarrell-Ash quarter-meter grating instrument l4 vhereinabove set forth.Thus, the instant system permits observation and recording of theconcentration of a plurality of pollutant gases without any requiredchanging of the correlation masks of the prior art.

Second, the determination of a pollutant gas concentration is strictlylinear function of the signal intensity from the detector of the instantinvention. The output signal from detector 17 is a measure of thecurvature in the spectral intensity curve expressed as a function ofwavelength, and it varies strictly linearly with concentration of theabsorbing gas in the radiant energy path, while the output signal fromthe detector of Barringer or Harris is not a linear function of suchconcentration. When a prior art multi-slit mask is employed, eachdifferent pollutant gas presents a different relation betweenconcentration and signal strength, and when different light sources areused or if different pollutant concentrations are present in the gasbeing analyzed, different relationships between the concentration andsignal strength are encountered. The use of a single wobbling slit inaccord with the instant invention produces a signal strength strictlyproportional to the pollutant concentration throughout the range of thegrating instrument 14. Thus, a single observation of the Signal outputfor a known concentration of gas is all that a person needs in theinstant system-for calibrating same, for any unknown concentration ofthat gas, or for any light source that may be used in the instantsystem, whether the unknown concentration of that gas is present in theabsorption cell 16 by itself or with any other pollutant gases in thesample being analyzed. While the Y Barringer, Harris and the instantinvention can all function to determine the presence or absence of aparticular gas (if a plurality of correlation masks are used) theBarringer and Harris systems do not provide a measure of theconcentration of such gas without the use of a calibration curveobtained byuse of a plurality of calibration points, or even acomparison of a standard sample for any given measurement, since therelationship between the gas concentration and the output signals is notlinearly related; nor is any simple calibration technique known whichwould provide reasonably accurate concentration readings; i.e., withoutusing a plurality of calibration points to construct a calibration curvefor each mask and different gas for each light source used in theBarringer and Harris systems.

An exemplary embodiment of the circuit components of FIG. 3 is seen toinclude the following parameters:

Solenoid coils 44 and 45 64 mh Capacitors 52 and 58 i pf 66 5 sfResistors 56 l8.2K ohms 65 and 68 lOOK ohms 67 2K ohms 69 464K ohms 70150 ohms Transistors 54 Fairchild 2N3565 60 RCA SK3009 Power Source 55i2 v DC Bar magnet 43 Alnico In the specific embodiment of the inventionconstructed and tested in accord with the invention, the Wobbler slit 36was displaced approximately 1 millimeter peak to peak with thereciprocation or oscillation frequency being 45 cycles per second. Thesevalues are not crictical but have proven to be effective in thefunctioning and performance of the spectrometer system constructed inaccord with the invention herein disclosed.

While the invention has been described with respect to a certainspecific embodiment, it will be appreciated that many modifications andchanges may be made by those skilled in the art without departing fromthe spirit of the invention. It is intended, therefore, by the appendedclaims to cover all such modifications and changes as fall within thetrue spirit and scope of the invention.

What is claimed as new and what it is desired to secure by LettersPatent of the United States is:

1. In a spectrometer system including a radiant energy source, ascanning spectrometer, only a single entrance slit disposed between saidsource and said spectrometer, said source producing a focused radiantenergy beam passing through said entrance slit and into saidspectrometer, said spectrometer having a single fixed exit slit wherebya radiant energy path is established from said source through saidentrance slit and through said spectrometer and out said exit slit, saidsystem including a photomultiplier detector in said radiant energy pathaft of and spaced from said exit slit, an absorption cell adapted tocontain a medium to be investigated, said cell communicating with saidradiant energy path between said exit slit and said detector,

said system having no correlation mask correspondingto a medium to beinvestigated in said cell, the improvement comprising means for movablymounting said entrance slit, said means including means forrectilinearly reciprocating said entrance slit at a constant eterwhereby radiant energy interference on said grid wire is non-existent.

3. In the system as defined in claim 1 wherein said detector receivesthe radiant energy after passage through an absorbing medium to beinvestigated in said cell and converts the received energy into amodulated signal, a tuned phase lock-in amplifier means for receivingsaid signal and converting said signal into a first signal of selectedfrequency and a dc signal, said amplifier means being tuned to the samefrequency as the frequency at which said entrance slit is reciprocating.

4. In the system as defined in claim 1 wherein said detector receivesthe radiant energy after passage through an absorbing medium to beinvestigated in said cell and converts the received energy into amodulated signal, a tuned phase lock-in amplifier means for receivingsaid signal and converting said signal into a first signal and rapidfrequency to modulate the wavelength of the radiant energy beam detectedin a sinusoidal manner.

2. In the system as defined in claim 1 wherein said detector includes aphotomultiplier tube having grid wires, said movable entrance slit beinglocated in the radiant energy path between said source and spectromofselected frequency and a dc signal, said amplifier means being tuned totwice the frequency as the frequency at which said entrance slit isreciprocating.

5. In the system as defined in claim 1 wherein said means forrectilinearly reciprocating said entrance slit includes circuit meansfor maintaining constant the amplitude of the reciprocating motion ofsaid entrance slit.

6. In the system as defined in claim 5 wherein said circuit meansincludes a selectively adjustable means for altering the amplitude ofthe reciprocating motion of said entrance slit.

7. In the system as defined in claim 1 wherein said detector receivesthe radiant energy after passage through an absorbing medium tobeinvestigated in said cell and converts the received radiant energyinto a modulated signal, a potentiometric recorder, a tuned phaselock-in amplifier means for receiving said signal and converting saidsignal into a first signal of selected frequency and a dc signal, saidfirst and dc signals being coupled to said recorder to record the ratioof said first signal to said dc signal.

8. In the system as defined in claim 7 wherein said means forrectilinearly reciprocating said entrance slit includes means forproviding a control signal representative of the frequency and phase ofthe reciprocating motion of said entrance slit to said amplifier for usein rectifying said first signal.

9. In the system as defined in claim 8 wherein said amplifier means istuned to the same frequency at which said entrance slit isreciprocating.

10. In the system as defined in claim 8 wherein said amplifier means istuned to twice the frequency at which said entrance slit isreciprocating.

11'. In a spectrometer system including a radiant energy source, aspectrometer, only a single entrance slit disposed between said sourceand said spectrometer, said source producing a focused radiant energybeam passing through said entrance slit and into'said spec trometer,said spectrometer having a single fixed exit slit whereby a radiantenergy path is established from said source through said entrance slitand through said spectrometer and out said exit slit, said systemincluding a photomultiplier detector, an absorption cell adapted tocontain a medium to be investigated, said cell communicating with saidradiant energy path between said exit slit and said detector, saiddetector receiving said radiant energy after passage thereof throughsaid cell and producing an output signal, the improvement comprisingmeans for rectilinearly reciprocating said entrance slit at a constantand rapid frequency to modulate the wavelength of radiant energy beamdetected in a sinusoidal manner, a potentiometric recorder for producinga ratio of two signals both derived from said output signal of saiddetector whereby said ratio is invariant with changes in the intensityof 7 said source.

12. In a spectrometer system including a radiant energy source, ascanning spectrometer, said source producing a focused energy beampassing into said spectrometer, said system including a photomultiplierdetector, an absorption cell adapted to contain a medium

1. In a spectrometer system including a radiant energy source, ascanning spectrometer, only a single entrance slit disposed between saidsource and said spectrometer, said source producing a focused radiantenergy beam passing through said entrance slit and into saidspectrometer, said spectrometer having a single fixed exit slit wherebya radiant energy path is established from said source through saidentrance slit and through said spectrometer and out said exit slit, saidsystem including a photomultiplier detector in said radiant energy pathaft of and spaced from said exit slit, an absorption cell adapted tocontain a medium to be investigated, said cell communicating with saidradiant energy path between said exit slit and said detector, saidsystem having no correlation mask corresponding to a medium to beinvestigated in said cell, the improvement comprising means for movablymounting said entrance slit, said means including means forrectilinearly reciprocating said entrance slit at a constant and rapidfrequency to modulate the wavelength of the radiant energy beam detectedin a sinusoidal manner.
 2. In the system as defined in claim 1 whereinsaid detector includes a photomultiplier tube having grid wires, saidmovable entrance slit being located in the radiant energy path betweensaid source and spectrometer whereby radiant energy interference on saidgrid wire is non-existent.
 3. In the system as defined in claim 1wherein said detector receives the radiant energy after passage throughan absorbing medium to be investigated in said cell and converts thereceived energy into a modulated signal, a tuned phase lock-in amplifiermeans for receiving said signal and converting said signal into a firstsignal of selected frequency and a dc signal, said amplifier means beingtuned to the same frequency as the frequency at which said entrance slitis reciprocating.
 4. In the system as defined in claim 1 wherein saiddetector receives the radiant energy after passage through an absorbingmedium to be investigated in said cell and converts the received energyinto a modulated signal, a tuned phase lock-in amplifier means forreceiving said signal and converting said signal into a first signal ofselected frequency and a dc signal, said amplifier means being tuned totwice the frequency as the frequency at which said entrance slit isreciprocating.
 5. In the system as defined in claim 1 wherein said meansfor rectilinearly reciprocating said entrance slit includes circuitmeans for maintaining constant the amplitude of the reciprocating motionof said entrance slit.
 6. In the system as defined in claim 5 whereinsaid circuit means includes a selectively adjustable means for alteringthe amplitude of the reciprocating motion of said entrance slit.
 7. Inthe system as defined in claim 1 wherein said detector receives theradiant energy after passage through an absorbing medium to beinvestigated in said cell and converts the received radiant energy intoa modulated Signal, a potentiometric recorder, a tuned phase lock-inamplifier means for receiving said signal and converting said signalinto a first signal of selected frequency and a dc signal, said firstand dc signals being coupled to said recorder to record the ratio ofsaid first signal to said dc signal.
 8. In the system as defined inclaim 7 wherein said means for rectilinearly reciprocating said entranceslit includes means for providing a control signal representative of thefrequency and phase of the reciprocating motion of said entrance slit tosaid amplifier for use in rectifying said first signal.
 9. In the systemas defined in claim 8 wherein said amplifier means is tuned to the samefrequency at which said entrance slit is reciprocating.
 10. In thesystem as defined in claim 8 wherein said amplifier means is tuned totwice the frequency at which said entrance slit is reciprocating.
 11. Ina spectrometer system including a radiant energy source, a spectrometer,only a single entrance slit disposed between said source and saidspectrometer, said source producing a focused radiant energy beampassing through said entrance slit and into said spectrometer, saidspectrometer having a single fixed exit slit whereby a radiant energypath is established from said source through said entrance slit andthrough said spectrometer and out said exit slit, said system includinga photomultiplier detector, an absorption cell adapted to contain amedium to be investigated, said cell communicating with said radiantenergy path between said exit slit and said detector, said detectorreceiving said radiant energy after passage thereof through said celland producing an output signal, the improvement comprising means forrectilinearly reciprocating said entrance slit at a constant and rapidfrequency to modulate the wavelength of radiant energy beam detected ina sinusoidal manner, a potentiometric recorder for producing a ratio oftwo signals both derived from said output signal of said detectorwhereby said ratio is invariant with changes in the intensity of saidsource.
 12. In a spectrometer system including a radiant energy source,a scanning spectrometer, said source producing a focused energy beampassing into said spectrometer, said system including a photomultiplierdetector, an absorption cell adapted to contain a medium to beinvestigated, means for sinusoidally varying the wavelength of theradiant energy beam from said source into said spectrometer at aconstant and rapid frequency, said means including a singlerectilinearly reciprocating entrance slit between said source and saidspectrometer, said sinusoidally varying beam passing from saidspectrometer through said absorption cell onto said detector, means toreceive the output signal from said detector and provide an indicationof the existence and concentration of the medium to be investigated.